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Following the discovery of high-temperature superconductivity in the La–H system, we studied the formation of new chemical compounds in the barium-hydrogen system at pressures from 75 to 173 GPa. Using in situ generation of hydrogen from NH₃BH₃, we synthesized previously unknown superhydride BaH₁₂with a pseudocubic (fcc) Ba sublattice in four independent experiments. Density functional theory calculations indicate close agreement between the theoretical and experimental equations of state. In addition, we identified previously knownP6/mmm-BaH₂and possibly BaH₁₀and BaH₆as impurities in the samples. Ab initio calculations show that newly discovered semimetallic BaH₁₂contains H₂and H₃^–molecular units and detached H₁₂chains which are formed as a result of a Peierls-type distortion of the cubic cage structure. Barium dodecahydride is a unique molecular hydride with metallic conductivity that demonstrates the superconducting transition around 20 K at 140 GPa.